The present invention relates, in general, to the field of DC to AC inverters and DC to DC converters and methods for use with the same. More particularly, the present invention relates to a drive control circuit and method for such inverters and converters of especial utility in conjunction with one-transformer, over-driven, push-pull, transformer-coupled, transistor oscillators.
Numerous circuits for DC to AC inverters and related DC to DC converters have heretofore been disclosed in the prior art. Such circuits have virtually innumerable variations and applications in present technology. An exhaustive examination of the entire field of the various DC to AC inverters and their characteristics has been conducted and described by Merrill Palmer and Robert J. Haver in their monograph entitled "The ABC's of DC to AC Inverters" published in 1979 as Application Note AN-222A by Motorola, Inc., assignee of the present invention.
In analyzing these prior art inverters and converters, numerous tradeoffs have been necessary to achieve a specific desired performance while keeping complexity and parts count to a minimum to control cost. Of all such devices previiously available, the least costly to produce is the one-transformer self-oscillating feedback oscillator. By using a single transformer the number of most costly elements in the circuit is, at once, minimized. Utilizing a pair of push-pull connected transistors which receive oscillation drive from a feedback winding and are used to drive the transformer primary provides a further saving over driven or bridge inverters.
A decided disadvantage, however, has been that the transistor pair must be closely matched in characteristics particularly beta (H.sub.fe) and turnoff times. Any mismatch can lead to simultaneous conduction of the transistors or to "current hogging" by which imbalance the transformer core can become saturated with an undesirable net DC bias. Providing for such imbalance in these push-pull devices which utilize core saturation to control oscillation would, in turn, require a larger, more expensive transformer. Other interrelated difficulties have included the setting of the oscillator frequency which has generally been dependent primarily on the transformer characteristics.